diff --git a/python/BioSimSpace/Process/_plumed.py b/python/BioSimSpace/Process/_plumed.py
index f18d87995..60966630c 100644
--- a/python/BioSimSpace/Process/_plumed.py
+++ b/python/BioSimSpace/Process/_plumed.py
@@ -470,7 +470,8 @@ def createConfig(self, system, protocol, is_restart=False):
                 num_atoms = system._sire_object.molecule(molecule).nAtoms()
 
                 # Create the ligand record. Rember to one-index the atoms.
-                string += "lig: COM=%d-%d" % (idx+1, idx+num_atoms)
+                colvar_string = "lig: COM=%d-%d" % (idx+1, idx+num_atoms)
+                self._config.append(colvar_string)
 
                 # Create the center-of-mass record for the atoms that are used
                 # to define the funnel origin.